network security related question

[Vitorio Okio]

> Could it be that you have your router configured in a way that makes
> your Ubuntu machine the "default server"? Maybe it has a different name
> in your router software. If you have enabled this option you don't block
> anything with your NAT setup. Then everything received by the router
> will be sent to your Ubuntu machine unless you have an extra filter to
> block specific traffic.

no, this is not the case.